mp3enc: fix const correctness

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mp3enc.c

@@ -372,7 +372,7 @@ static int mp2_write_trailer(struct AVFormatContext *s)
 
 static int query_codec(enum AVCodecID id, int std_compliance)
 {
-    CodecMime *cm= ff_id3v2_mime_tags;
+    const CodecMime *cm= ff_id3v2_mime_tags;
     while(cm->id != AV_CODEC_ID_NONE) {
         if(id == cm->id)
             return MKTAG('A', 'P', 'I', 'C');